recovery process and system for a pulp mill

ABSTRACT

A method for burning chlorine-containing liquors in a chemical recovery boiler at a pulp mill, wherein the recovery boiler includes spent liquor sprayers for feeding spent liquor and a number of combustion air levels including: increasing a combustion temperature in the recovery boiler in a burning zone where a chlorine-containing liquor or a chlorine-containing effluent is burned; while burning the liquor or effluent, volatilizing the chlorine in the liquor or effluent to produce chloride-containing salts in flue gases in the boiler, and removing the chloride-containing salts from the flue gases.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/913,322, filed Apr. 23, 2007, the entirety ofwhich is incorporated by reference.

BACKGROUND OF INVENTION

This invention relates to recovery processes for processing naturalcellulosic or other fibrous material and, particularly, to the removalof chlorine from such processes.

Chlorine (Cl) is present in wood, in make-up chemicals and pulpbleaching filtrates, especially when using chlorine-containing bleachingchemicals. Chlorides entering with the wood and input chemicals tend tobuild up in pulping liquors. This may be a particular problem forcoastal mills where logs are transported in sea water and becomesaturated in chloride. Chemicals used in the process may also containconsiderable amounts of chlorine. The concentration of chlorine in blackliquor varies greatly from one process to another. The chlorine contentin black liquor can be as low as 0.1 to 0.8% of the liquor dry solids,but in some cases the chlorine content of the liquor dry solids can beas high as about 5%, and in closed cycle processes it may rise evenhigher.

One proposed technique for decreasing the environmental impact ofchlorine-containing chemicals is the closing of the liquid circulationsof bleaching plants, and modern bleaching plants have reached the levelof 10-15 m3/adt without any negative impact on the quality of the pulp.However, even when decreasing the amount of effluent from the level of15 m3 to the level of 10 m3, an increase in chemical consumption becomesvisible, which thus leads to an ever-increasing amount of organicchlorine compounds from bleaching. Thus, a conclusion can be made thatclosing the water circulations of bleaching does not as such have adirect effect on the amount of organic chlorine compounds, but on theother hand, a decreased amount and higher concentration of effluentsallows for an easier and more economical purification thereof.

Thus, the dominance of chlorine dioxide as bleaching chemical has gainedeven more foothold in the recent years, and even the most up-to-dateresearches and industrial experiences have been unable to undermine itsprominence, but as a rule the pulping industry, with only a fewexceptions, has accepted the use of chlorine dioxide as the key chemicalin bleaching. Therefore, if a mill is to further decrease the amount oforganic chlorine compounds, the aim in the mill will, above all, betheir elimination and their treatment inside the mill, rather than adecrease in the use of chlorine dioxide.

Modern ECF-bleaching used for bleaching pulp typically comprises atleast three bleaching stages and three washing apparatuses. In specialcases, only two washing apparatuses may be used, but there are only afew such applications in the world. All bleaching sequences using atleast one chlorine dioxide stage and not using elementary chlorine inany bleaching stage, are regarded as ECF-bleaching. Further, a bleachingsequence comprises one alkaline stage, which at present typically useseither oxygen, peroxide, or both as auxiliary chemical. Additionally,ozone, various types of acid stages and a chelate stage may be used inmodern bleaching for removing heavy metals.

When bleaching is referred to as ECF-bleaching, the amount of chlorinedioxide used therein is over 5 kg/adt, and even then the applicationsare referred to as so-called Light ECF applications. Typically,light-ECF applications make versatile use of the removal of hexenuronicacids, i.e. the A-stage (as described in U.S. Pat. No. 6,776,876);peroxide is used in one or two stages, and in some cases also an ozonestage. The total amount of chlorine dioxide varies from the mentioned 5kg/adt up to a level of about 25 kg/adt. If chlorine dioxide is used inone bleaching stage, the charges are most typically between 5-15 kg/adtand if the mill is provided with two chlorine dioxide stages, chargesless than 10 kg/adt are rarely used.

If the use of peroxide in bleaching is limited to charges below 6 kg andif chlorine dioxide is the main bleaching chemical, the chlorine dioxidecharge in the bleaching increases from a level of 25 kg/adt depending onthe bleaching properties of the pulp in question and on the level ofdecrease in the kappa number before starting the bleaching withchlorine-containing chemicals. Therefore, bleaching technique can, inview of the process, be fairly freely adjusted to various chlorinedioxide consumption levels in such a way that the amount ofchlorine-containing chemicals exiting the bleaching corresponds to thecapability of the chemical circulation to receive chlorides.

When chemical liquor cycle in kraft mills will be more closed, chloride,potassium, metals and other Non Process Element (NPE) concentrations inthe liquor cycle are increased.

In chemical pulp mills, the chemicals of a pulping process are recoveredfrom spent liquor, e.g., black liquor in kraft pulping, by firing theliquor in a recovery boiler either alone or together with other “waste”streams. The firing process is exothermic and the released energy isrecovered as pressurized superheated steam. The steam energy isrecovered in a steam turbine in the form of electric power and steam ofdifferent pressures for process needs. In the recovery boiler, chlorineand potassium are enriched into the fly ash and increase thecorrosiveness of the flue gas especially in the superheater.

Improved methods of handling chlorine-containing liquors and effluentsat pulp mills so that corrosion problems and other adverse effectscaused by chlorine can be minimized.

CA 2041536 describes a treatment of a DC-stage effluent in a specialevaporator and incinerator without recovering valuable chemicals fromthe incineration ash. U.S. Pat. No. 5,374,333 relates to a process inwhich all liquid effluents from a bleach plant are evaporated andincinerated independent of the recovery boiler to produce a residuecontaining sodium, sulfate and carbonate, which residue is leached toproduce a leachate. At least a substantial portion of the leachate isfed to the chemical recovery loop associated with the recovery boiler.

EP 719359 describes a process in which liquid effluents from a bleachplant are concentrated and incinerated in a recovery boiler to produceflue gases including ash containing salts including sodium, potassium,and chloride-containing salts, and sulfur compounds. Potassium andchloride are removed from the ash while returning the sulfur containingcompounds of the ash to the recovery loop, so as to balance the sulfur,chloride and potassium levels in the mill.

U.S. Pat. No. 5,989,387 discloses a method for reducing the chlorineconcentration in a sulfate cellulose process, wherein part of thechlorine content in the chemical cycle is separated from the cycle andremoved. In this process sulphurous odour gases are introduced into thesoda recovery boiler at least in such an amount that the concentrationof sulphur oxides in the soda recovery boiler is such that at least partof chlorine separating in gaseous form from the bed is in the form ofhydrogen chloride in the upper part of the soda recovery boiler. Thehydrogen chloride is separated from the flue gases by scrubbing the fluegases.

The above described conventional methods do not address burningchlorine-containing effluents in a recovery boiler such that chlorinecould be removed from the chemical recovery loop efficiently.

SUMMARY OF THE INVENTION

A process has been developed and is disclosed herein for treating spentliquors and filtrates or effluents from bleaching using chlorine dioxideat a pulp mill and for removing chlorine (Cl) from the process. Thisallows high water reuse and effective production of power and heat fromspent liquor, such as black liquor, and other energy-containing streamsavailable at the mill, or brought to the mill. The process disclosedherein can also be used for balancing and stabilizing the Clconcentrations in the material circulations of the mill, specially theconcentration level in the spent liquor, when chlorine enters the millin raw materials and chemicals streams. The process disclosed hereinpreferably relates to sulfate or Kraft pulp mills.

A process has been developed in which the burning of chlorine-containingliquor and effluents can be controlled in such a way that the operationof the recovery boiler itself is efficient, whereby a high-temperaturesteam can be produced for power and heat production. The developedprocess may allow for chlorine to be separated efficiently and for thechlorine level of the pulp mill can be balanced without adverselyaffecting the pulp quality and the operation of the recovery boiler.Especially corrosion problems in the machinery can be avoided orminimized.

A method is disclosed herein for burning chlorine-containing liquors ina chemical recovery boiler at a pulp mill, wherein the recovery boilercomprises spent liquor sprayers for feeding spent liquor and a number ofcombustion air levels. A feature of the disclosed method is that thecombustion temperature in the recovery boiler is increased in a zone,where a chlorine-containing liquor or effluent is burned, for improvingthe volatilization of chlorine from the liquor or effluent into fluegases to produce chloride-containing salts, and that the flue gases aretreated to remove the chloride-containing salts. The chlorine-containingstream to be burned is typically a spent liquor, such as black liquor,from pulp production or a chlorine-containing effluent from a bleachingplant of the pulp mill. Also other chlorine-containing streams from thepulp mill can be treated according to the process disclosed herein.

According to an embodiment of the method disclosed herein, at least 30%calculated from the as fired liquor chlorine concentration isvolatilized into the flue gases. Preferably over 40% chlorine deliveryfrom the as fired stream chlorine concentration into flue gases isobtained by adjusting the combustion zone temperature high enough.

According to an embodiment of the method disclosed herein, the oxygenconcentration in the recovery boiler is increased in the burning zone ofthe chlorine-containing stream for raising the temperature of the zone.

According to an embodiment of the method disclosed herein, the pulp millhas a bleach plant using chlorine dioxide, and the bleach plant has atleast one chlorine dioxide stage, and chlorine-containing effluent flowfrom the bleach plant is concentrated and burned in the recovery boiler.

According to an embodiment of the method disclosed herein, oxygenenrichment takes place at the primary and/or secondary air levels of thecombustion air. Preferably oxygen enrichment takes place at secondaryair level or levels.

According to an embodiment of the method disclosed herein, the recoveryboiler is provided with an integrated separate combustion chamber, wherethe chlorine-containing liquor is burned. Typically thechlorine-containing stream that is burned in the separate combustionchamber is a bleaching effluent.

According to an embodiment of the method disclosed herein,oxygen-enriched air is added to the zone where the stream having thehighest chlorine concentration is burned.

According to an embodiment of the method disclosed herein,oxygen-enriched air is added to the integrated combustion chamber.

According to an embodiment of the method disclosed herein, the oxygencontent is increased by raising the oxygen content of the combustion airsupplied to the burning zone.

According to an embodiment of the method disclosed herein, the oxygencontent in the boiler is increased by supplying oxygen directly to theburning zone.

According to the methods disclosed herein, the temperature in acombustion zone where a chlorine-containing liquor or effluent is burnedis increased so that the delivery of chlorine from the liquor into fluegases formed in the burning is maximized. Thus, the chlorinevolatilization and pyrolysis take place in the zone where the liquor isburned. The combustion zone temperature is over 800° C., typically over850, preferably over 950° C., most preferably over 1150° C.

FIG. 3 shows a chart of the operation of a kraft recovery boiler inwhich the proportion (r) of Cl, calculated based on the Cl amount in asfired black liquor, found in flue gases, as the function of furnaceloading (MW/m² bottom area of the furnace). The upper line represents anoperation model having a higher temperature in the combustion zone, thelower line represents an operation model having a lower temperature inthe combustion zone. This shows that raising the combustion zonetemperature can increase the chlorine volatilization from the as firedstream into flue gases. This result is utilized in the presentinvention. Chlorine concentration into flue gases is maximized viaincreasing the combustion zone temperature. The proportion (r) can beincreased with high dry solids of the spent liquor (80-90%), with firingintensity, with proper air distribution, and/or with high airtemperature and/or with addition of oxygen to the furnace, preferablyclose to the point where the stream having the highest chlorine (Cl)concentration is fed to the furnace. Thus one suitable way to increasethe combustion zone temperature is to have stoichiometric conditions orclose (the air factor is 0.85-1.0, preferably 0.9-1.0). in thecombustion zone. This can typically be achieved by proper combustion airdistribution or addition of oxygen. In the process disclosed herein thecombustion zone temperature for a chlorine-containing liquor or effluentis raised intentionally so as to increase or maximize chlorinevolatilization from as fired stream into flue gases. Chlorine compoundscan then be removed from the flue gases by a suitable process. Theremoval of chlorine may be practiced according to many conventional orknown techniques, such as evaporation/crystallization

By increasing the combustion temperature e.g. by optimizing combustionintensity/m² it is possible to volatilize, over 30%, preferably over40%, calculated from the as fired spent liquor Cl concentration, intoflue gases, typically as sodium chloride (NaCl) or potassium chloride(KCl). Even more than 50% Cl from the as fired streams can be deliveredinto flue gases—in theory 100%, but not in practice. In principlechlorine could also be in form of hydrogen chloride (HCl), but HCl isfavoured by low furnace temperature, which results into low delivery ofCl into flue gases. HCl could be washed out from flue gases, as isknown.

Further increases in chlorine delivery into flue gases from thechlorine-containing stream can be achieved with the use of a separatecombustion chamber integrated with a recovery boiler. The combustiontemperature for the chlorine-containing stream, typically bleachingeffluent, in the chamber can be increased with the use of oxygen oroxygen enriched air. Further the burning in the chamber can be improvedby a high flame temperature producing combustion agent such as fuel oil,natural gas, methane, ethanol, methanol, other biofuels and chemicals,which are included in the mill processes. The chamber may have thermalinsulation or brickwork to increase the combustion temperature in thechamber.

When the processes disclosed herein is used for balancing chlorine levelin spent liquor, the chlorine concentration entering the boiler furnacemay be so high that under a traditional arrangement high live steamtemperature, or live steam and reheated steam temperatures cannot beachieved without corrosion—under reasonable costs. In that case aprocess can be applied in which the recovery boiler is provided with aseparate combustion cavity or chamber having a heat exchanger for finalsuperheating of the steam produced in the superheater section of therecovery boiler, whereby the heat exchanger is connected to thesuperheaters of the boiler. The cavity is heated by burning fuel in sucha manner that non-corrosive conditions in the combustion chamber areguaranteed. The fuel used in the combustion chamber can be gas producedfrom biomass, liquefied biomass, methanol, other biofuels, natural gas,LPG, etc. The criterion for the fuel is the non-corrosive nature underthe combustion chamber conditions.

Thus the recovery boiler used in connection with the processes disclosedherein can be provided with a separate combustion chamber for burning achlorine-containing stream or for final superheating of steam from thesuperheater section of the boiler, or for both purposes. In the lastmentioned alternative the recovery boiler has at least two separatecombustion chambers, one for burning a chlorine-containing stream andone for final superheating of steam from the superheater section of theboiler.

Chlorides and potassium are enriched in the recovery boiler ash. Cl andK can be removed from the ash by methods known per se, such as leaching,evaporation/crystallization, freeze crystallization. One preferableprocess for ash handling is described in connection with FIG. 1.

SUMMARY OF THE DRAWINGS

The process that has been developed will be described in more detailwith reference to the attached drawings, in which:

FIG. 1 illustrates a schematic illustration of the basic components ofone exemplary system that incorporates the developed process andarrangement;

FIG. 2 is a schematic view of a recovery boiler system with anintegrated combustion chamber; for high electricity production.

FIG. 3 depicts, in graphic form, a proportion of chlorine volatilizedfrom as fired spent liquor into flue gases in Kraft recovery boilers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The exemplary system illustrated in FIG. 1 includes a cooking plant 2which typically comprises a digester, such as a continuous digester, towhich hard wood or soft wood chips, or other comminuted cellulosicmaterial, is fed through line 1. In the digester the wood chips areacted upon by the cooking chemicals at temperature and pressureconditions so as to produce chemical cellulose pulp, such as kraft pulp.The pulp is further typically treated in brown stock washing and in ascreening room. Then the pulp is preferably subjected to oxygendelignification in stage 3. After oxygen delignification, the pulpproceeds to the bleach plant where it is subjected to bleaching in aplurality of different bleaching stages. The particular bleaching stagesthat are utilized can be varied, and are also dependent upon theparticular cellulose material being treated, but in at least onebleaching stage chlorine dioxide is used as a bleaching chemical.Typical sequences are A/D-EOP-D-P and D-EOP-D-P. In FIG. 1 a D stage isafter oxygen delignification 3, but there can be other stages beforeD-stage 4 including washing which is shown as an example only. Chlorinedioxide in line 6 is added to stage 4, and after that washing liquid,such as water through line 7. The pulp is passed to a further treatmentvia line 5.

Weak black liquor from the cooking plant 2 is passed in line 21 toevaporator 25, 22 where it is evaporated to a concentrated black liquorin line 18 to be fired in the recovery boiler. Dry solids concentrationof the weak black liquor is typically 12-17%, and the firing liquorconcentration respectively 75%, preferably 80-85%. The evaporator ismost often a multiple effect evaporator with water evaporation of 6-12ton/ADT. Primary steam 19 is introduced into the first evaporator effectwhere part of the water in the black liquor is vaporized. The vapor isthen used as heating steam in the second effect, which is operated atlower pressure and temperature than the first effect. Similarly thevapors are introduced into the subsequent effects and finally the vaporfrom the last effect 22 is condensed in a surface condenser (not shown)or the vapor in line 23 is used as heating steam for bleach planteffluent evaporator, 9. Multiple effect evaporators have typically 5-8effects and the primary steam consumption is respectively 2.2-0.8ton/ADT.

Evaporated water vapor contains also some methanol and volatile organicsulfur compounds but practically no inorganic compounds. The vapors canbe fractionated and stripped to clean secondary condensate 24 which canbe used as process water in fiber line processes, such as at 3. Cookingchemicals and dissolved organic and inorganic solids from wood (e.g.chlorine, heavy metals like cadmium and lead) remain in the concentratedblack liquor in line 18.

Chlorine-containing effluent 8 from the acidic bleaching stage 4 isconcentrated e.g. in a multiple effect evaporator 9. The effluent flowis typically 3-5 m3/ADT having 0.2-1% dissolved dry solids (e.g.chloride and heavy metal ions). The effluent is evaporated toconcentrations of 5-20% or even to higher concentrations. Theconcentrate 10 is fired in the recovery boiler 17. Depending on therequired evaporation capacity the effluent evaporator 9 can utilizesecondary vapors (23) from the black liquor evaporator back end stages22 or primary steam 19, also mechanical vapor recompression type ofevaporator can be used.

The concentrated spent liquor from pulping in line 18 is fed into thefurnace 43 via liquor spraying devices 16. The liquor stream in line 18may be divided and introduced at several levels 15 into the recoveryboiler furnace. These different locations are situated on a front wall,rear wall and sidewalls. The spent liquor burns in the furnace, ascombustion air is available from several air feed points. One typicalspent liquor is called black liquor, from kraft pulping, which is burnedand the chemicals recovered in a so called kraft recovery boiler. In akraft recovery boiler the combustion air is fed into the boiler viaseveral air ports at several levels, which are primary air, at thelowest air port level(s) 46′ at the lower part of the furnace, secondaryair level or levels, 46, above the primary air level but below theliquor nozzles, and tertiary air level or levels, 44, above the liquornozzles to ensure complete combustion. Sometimes the highest tertiaryair level is called a quaternary air level. Combustion airs may containweak odorous gases from the pulp mill, and/or from the recovery boiler.Oxygen or oxygen enriched air in line 45 is fed into the furnace. In EPPatent 953080 a method is described in which oxygen enriched air is fedto the lower furnace of a recovery boiler so that the air factor islowered, which contributes to e.g. increase in the firing capacity ofthe boiler.

The spent liquor 18 contains typically at least some chlorine (Cl), forinstance 0.05-2% based on the dry solids analysis. The concentratedbleach plant effluent flow 10, which contains typically a higher Clconcentration, based on dry solids, than flow 18, is also fed into therecovery boiler furnace 43. The feeding place or places 11 may belocated in the same zones and at the same levels where the sprayingdevices 16 are located.

Alternatively, the flow 10 may be fed with spraying, or through a burneror burners via line 50 into a separate combustion chamber 49, which isintegrated into the furnace 43 of the recovery boiler, and from whichchamber flue gases enter the furnace 43.

In principle the arrangement is similar to that shown in FIG. 2 anddisclosed in U.S. Patent Applications Nos. 2006-236696 or 2005-252458.

The additional combustion chamber 49 of the recovery boiler is locatedprior to superheaters 41, and prior to reheaters (not shown), whenfollowing the flue gas path 17 from the recovery boiler furnace 43. Thechamber 49 may have thermal insulation or brickwork to increase thecombustion temperature in the chamber. The flue gases from the chambermay enter the furnace 43 flowing down or flowing up.

The flows 10 and 18 can be fed separately or mixed prior to the recoveryboiler 43, or inside the evaporation plant 25, and the mixed flow can befed into the furnace via devices 16. The main part of the inorganics inspent liquor, typically cooking chemicals, chemicals for the fiber line,or chemicals for energy or special chemicals production, are dischargedfrom the lower furnace, as smelt in line 14, or recovered from fluegases 38 in a separation device such as electrostatic precipitator 36into stream 35 to be further processed into 26.

In kraft pulping a chemical smelt 47 is formed on the bottom 48 of thefurnace of the recovery boiler. The smelt flow 14 enters dissolving tank13 for further recovery and preparation of cooking chemicals. Prior artdescribes various processes for the green liquor handling and caustizing12, including removal of undesired components, such as heavy metals.

A solution has been developed for effective chlorine removal fromrecovered streams, i.e. chemical melt 14 formed in the recovery boiler,and stream 26 including sodium sulfate and sodium carbonate from ashhandling, comprising the following:

Cl concentration into flue gases 17 is maximized via increasing thecombustion zone temperature where Cl containing streams 10 and 18 areburned. The proportion of Cl, calculated based on the Cl amount in asfired black liquor, found in flue gases can be increased with high drysolids of the spent liquor, with firing intensity, with proper airdistribution, and/or with high air temperature and/or with addition ofoxygen to the furnace, preferably close to the point where the streamhaving the highest chlorine (Cl) concentration is fed to the furnace.

By optimizing e.g. combustion intensity/m² it is possible to volatilize,at least over 30%, calculated from the as fired spent liquor Clconcentration, into flue gases, typically as sodium chloride (NaCl) orpotassium chloride (KCl). The delivery of Cl into flue gases isincreased via the use of oxygen or oxygen enriched air 45. If thefurnace temperature is high enough, more than 40%, or even more than 50%Cl delivery from the as fired streams can be delivered into fluegases—in theory 100%, but not in practice.

Further increase in Cl delivery into flue gases from the concentratedbleaching effluent stream 10 in line 50 can be achieved by using theintegrated combustion chamber described above, in which the burning isintensified with the use of oxygen enriched air via line 51. Further theburning in the chamber can be improved by a high flame temperatureproducing combustion agent as fuel oil, natural gas, methane, ethanol,methanol, other biofuels, chemicals, which are included in the millprocesses.

Adequately high sodium (Na) and potassium (K) volatilization from thespent liquor combustion is required for binding Cl into NaCl and KCl.Also for Na and K a higher combustion temperature increases deliveryinto flue gases 17. In the furnace NaCl and KCl are formed, and theyturn into fine particles, ash, which deposit onto heat transfer surfaces41 and 39. The main part is captured as fly ash in the precipitator 36to be processed, stream 35. The main part of the ash is, however, formedof useful SO4 and CO3 salts. If the chamber described above is locatedin the upper part of the furnace 43, part of Cl may enter precipitator36 as gas, HCl, which can be removed from flue gases 37 exiting theprecipitator by using known technology, such as scrubbing.

Flue gases from the recovery boiler 43 contain inorganic dry solidsparticles, which are separated in electrostatic precipitator 35. Themain components in the ash are sodium sulfate and sodium carbonate. Theash contains also potassium salts, chlorides and several metals, such ase.g. cadmium and lead, which are easily vaporized in the recovery boiler43, The ash amount is typically 6-12% of the dry solids fired in therecovery boiler, equal to about 80-200 kg/ADT. The ash is returned backto the evaporator or to the firing liquor to recover valuable chemicals.

Chloride and potassium are enriched in ESP ash and therefore chlorideand potassium are favorably removed from the ash. The ash is dissolvedin hot water or condensate 34, in mixing tank 33, and thenrecrystallized in evaporator crystallizer 27. Valuable sodium sulfateand carbonate are first crystallized and separated from the motherliquor and after the separation the crystals 26 are fed back throughline 20 to black liquor evaporator 25. The mother liquor in line 28 richin chloride and potassium is purged to sewer or may be further utilizedin processes developed for that purpose.

While dissolved ash solution in mixing tank 33, is alkaline, pHtypically 10-11, the metal ions in the ash are insoluble forming finemetal hydroxide particles in the solution. The particles are separatedfrom the solution 32 in the filter or in other separation equipment, 30,and the filter cake is led to further treatment, 31. The filteredsolution, 29, is led further to the ash recrystallizer, 27.

When the process is used for balancing Cl level in spent liquor, the Clstream entering the boiler burning streams, spent liquor in line 105(FIG. 2) and optionally bleaching effluent in line 101 mixed with thespent liquor, may be so high that under traditional arrangement highlive steam temperature, or live steam and reheated steam temperaturescannot be achieved without corrosion—under reasonable costs. In thatcase a system can be applied in which a combustion cavity or chamber isprovided in connection with a recovery boiler for the final superheatingof steam produced in the superheater section of the recovery boiler, asshown in FIG. 2 or described for example in US 2005-252458. The systemallows heating the steam in the conventional heat transfer sections(i.e. economizers, boiler bank, and superheaters) of the recovery boilerinto such a degree that high temperature corrosion does notsubstantially take place, i.e. below 520° C., optimally 480-500° C., andafter that the steam is final superheated to 500-600° C., optimally to520-560° C. in the combustion cavity, which serves as a finalsuperheater. Thus the chamber can also be used for final increase of thetemperatures of live steam and reheat steam, if the flue gases 126generated in the recovery boiler furnace are too corrosive for finalsuperheating and reheating. The corrosiveness of Cl and K increase withtemperature. The corrosiveness of Cl and K impose an upper temperaturelimit on the steam generated in the recovery boiler. This limit for thesuperheated steam temperature is typically 400° C. to 490° C., dependingon the chlorine and potassium content. However, the target uppertemperatures for the steam are typically up to 520-560° C. or higher, asmentioned above. The fuel for superheating of steam is preferably thenoncorrosive nature under the conditions of the combustion chamber. Inthis case the fuel used in the chamber is preferably a biofuel. The fuelcan be a gas produced by gasifying biomass. Instead of the gas producedfrom biomass other fuels can be used, e.g. liquefied biomass, methanol,ethanol, natural gas, LPG etc.

In FIG. 2, the cavity 102 may comprise a single chamber or a pluralityof cavities that are arranged in parallel and/or serial. The cavity mayshare a wall with the furnace 103 and the walls of the cavity may bewater-cooled. Combustion gases generated in the cavity 102 flow into thefurnace as additional flue gases 127. The cavity may include asuperheater 113. Superheated steam flows via steam conduit 123 from theconventional superheaters 108 in the boiler to the superheater(s) 113 inthe cavity 102 (or cavities). The cavity 102 may include one or moreburners 125. Flue gases 127 formed in the cavity enter the furnace andcombine with the flue gases 126 in the furnace of the recovery boiler.Combustion air 128 is injected into the cavity 102 to promote combustionin the burners 125. The burners 125 generally burn gas fuel generated ina gasifier 129 and that flows via gas supply conduit 130. The gasgenerated by the gasifier 129 may be distributed via line 131 for otherpurposes in addition to providing fuel for the cavity burners 25. Thegas from the gasifier may be cleaned or otherwise treated in a gastreatment device 132 before flowing to the burners.

In connection with the disclosed process, combustion chambers integratedinto the recovery boiler can be used for burning concentrated bleachingeffluents and/or for final superheating of the steam from the recoveryboiler.

The process may increase the investment costs of chemical circulation,therefore it is reasonable to set such guidelines for the bleaching thatthe investment costs thereof can be controlled. It is thereforereasonable to select bleaching sequence A/D-EOP-D-P with four bleachingstages as a reference sequence and to exclude ozone. For softwood, thecorresponding sequence is D-EOP-D-P. In this case, the quality of thepulp can be considered to correspond to the properties of ECF-pulp andthe yield remains reasonable. This way, the chlorine dioxide charge forsoftwood is between 25-35 kg/adt and for hardwood 20-30 kg/adt. Theseparameters can be regarded as rating values, and thus no new techniquesneed to be invented for bleaching.

When the amount of active chlorine is calculated as the amount ofchlorides in the way described above, it is noted that for softwood, ableaching line produces, in order to obtain good bleaching results,about 10 kg of chlorides per one ton of cellulose, and a hardwoodbleaching line even less. If the mill is closed in such a way that lessand less fresh water is introduced into the bleaching, as much as 50%larger chlorine dioxide charges may be expected, and on the other handthe amount of chlorides in bleaching effluents will increase up to alevel of 15 kg. Levels higher than this cannot be consideredeconomically reasonable, but the main idea of the bleaching correspondsto these basic solutions. By means of the disclosed process andarrangement, the chlorine/chloride concentrations in different parts ofthe pulping and recovery processes can be controlled.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements.

1. A method for burning chlorine-containing liquors in a chemicalrecovery boiler at a pulp mill comprising: increasing a combustiontemperature in a burning zone of the recovery boiler, where achlorine-containing liquor or a chlorine-containing effluent is burned;while burning the liquor or effluent, volatilizing the chlorine in theliquor or effluent to produce chloride-containing salts in flue gases inthe boiler, and removing the chloride-containing salts from the fluegases.
 2. The method of claim 1 wherein over 30 percent, as calculatedfrom as fired liquor chlorine concentration, is volatilized into theflue gases.
 3. The method of claim 1 wherein over 40 percent of chlorinefrom as fired stream chlorine concentration is volatilized into the fluegases.
 4. The method of claim 1 wherein the pulp mill includes a bleachplant using chlorine dioxide, and the bleach plant has at least onechlorine dioxide stage, and chlorine-containing effluent flow from thebleach plant is concentrated and burned in the recovery boiler.
 5. Themethod of claim 1 wherein the chlorine-containing liquor is a spentliquor.
 6. The method of claim 5 further comprising mixing a bleachingeffluent with the spent liquor before supplying the chlorine-containingliquor to the recovery boiler furnace.
 7. The method of claim 5 furthercomprising superheating steam from a superheater of the recovery boilerin a separate combustion chamber of the boiler.
 8. The method of claim 5wherein the recovery boiler is provided with at least two separatecombustion chambers, said method further comprises burning achlorine-containing stream in one of the two separate combustionchamber, and superheating steam from a superheater of the recoveryboiler in another of the two separate combustion chambers.
 9. The methodof claim 1 wherein the recovery boiler is provided with an integratedcombustion chamber, and the chlorine-containing liquor is burned in theintegrated combustion chamber.
 10. The method of claim 1 furthercomprising increasing an oxygen concentration in the burning zone toincrease the combustion temperature.
 11. The method of claim 10 whereinthe oxygen concentration is increased by adding oxygen-enriched air to alocation adjacent to where the liquor or effluent is fed to the boiler.12. The method of claim 10 wherein the oxygen concentration is increasedby raising an oxygen content of combustion air supplied to the burningzone.
 13. The method of claim 10 wherein the oxygen concentration isincreased by supplying oxygen directly to the burning zone.
 14. Themethod of claim 10 wherein oxygen-enriched air is added to a combustionchamber of the boiler.
 15. The method of claim 10 wherein theoxygen-enriched air is added through at least one secondary air level ofthe recovery boiler furnace.
 16. The method of claim 1 wherein a highdry-solids content of the liquor or effluent contributes to the increasethe combustion temperature.
 17. The method of claim 1 wherein thecombustion temperature is increased by setting a combustion firingintensity in the burning zone.
 18. The method of claim 1 wherein thecombustion temperature is increased by adjusting a distribution ofcombustion air in the recovery boiler.